Active source signals are sometimes used for discovery and/or analysis of (e.g., imaging of) objects that are obstructed from view. Active source signals, as used herein, refer generally to signals that are input to a target site, penetrate an obstructing medium (e.g., which is obstructing the view of the object that is of interest), and at least a portion of the active source signals may be reflected and captured by receivers for analysis. The captured reflected signals may be processed to discover and/or analyze (e.g., image) the object that is of interest. Thus, an active source signal is one that is transmitted into a target site, and at least a portion of such active source signal may be reflected by object(s) present in the target site, whereby the reflected signal may be captured by receivers and processed to analyze (e.g., image) the object(s) present in the target site. Accordingly, the active source signals are input to a target site, and the reflected portion(s) of such active source signals are information that is desired for analyzing (e.g., imaging) the object(s) present in the target site.
Examples of active source signals include various types of force or pressure signals. One example of active source signals includes force or pressure signals (or “waves”) commonly referred to as seismic waves, such as are commonly used in seismic exploration applications. Another example of active source signals includes acoustic signals, such as are commonly used in sonar applications (e.g., submarine navigation), ultrasound applications (e.g., medical imaging, such as sonography), etc.
In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon and/or other mineral deposits. Generally, a seismic energy source is used to generate a seismic signal (or “wave”) that propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
Currently, there are few options for operators of oil and gas recovery processes to monitor or image the distributions of fluids and solids during the recovery process (e.g., during operation of a well). As a recovery process proceeds, imaging of its current state, i.e. its distributions of pressures and phase saturations, is essential to understand where hydrocarbon (e.g., oil and/or gas) pockets remain in the reservoir to maximize the opportunity to recover the resource with less uncertainty. Imaging a reservoir is difficult because often reservoirs are greater than 300 meters (m) deep, and typically greater than 1,000 m deep.
As mentioned above, one such option is reflection seismic imaging where a large impulse-sounds signal is imposed at the surface or from a well and reflections of the sound waves are used to build an image of the reservoir environment as well as rock layers above and below the reservoir. This makes it possible to “see” the location of the reservoir, potential gas zones, faults, and other features of the underground system. Traditional reflection seismology is intensive because it requires many people in the field and interpretation of the reflections is often subjective and can take weeks to months to process. Seismic imaging is also limited because much of the signal is lost and thus the reflections are degraded during the seismic “shoot”. Also, the sound waves have wavelengths of orders of 10 m and higher, thus only features larger than this size scale can be seen within the rock.
There are two traditional modes for seismic monitoring: 1) passive and 2) active. With passive monitoring, listening devices, commonly referred to as geophones, are placed into the ground for listening (i.e., receiving acoustic signals) from a target site. In this passive mode, no active source signals are input to the target site for the purpose of generating reflections for analysis of the site, but instead listening devices merely passively listen for any acoustic signals coming from the target site.
In active monitoring, an active source signal is input to the target site for purposes of generating reflected signals for receipt by receivers and subsequent processing of the reflected signals for analysis of the target site. Traditional active techniques typically require quieting operations at the target site. That is, it is traditionally desirable to minimize/eliminate external interference sources that may impart interfering signals to the target site during the time that active monitoring is taking place. Thus, in a seismic analysis application, wells or other equipment operating at a target site for extracting subterranean hydrocarbon reserves are stopped and quieted during the time of the active monitoring. This quieting is generally desired to minimize interference signals and make it easier to correlate received reflected signals with the active source signals that are input to the target site. Accordingly, active monitoring techniques are traditionally not performed in real-time time during operation of equipment that is otherwise operating at a target site for other purposes, such as for extraction of hydrocarbon reserves in a seismic application.
Various sources of seismic energy have been utilized in the art to actively impart seismic waves into the earth. Such sources have included two general types: 1) impulsive energy sources, such as dynamite, and 2) seismic vibrator sources. The first type of geophysical prospecting utilizes an impulsive energy source, such as dynamite or a marine air gun, to generate the seismic signal. With an impulsive energy source, a large amount of energy is injected into the earth in a very short period of time. Accordingly, the resulting data generally have a relatively high signal-to-noise ratio, which facilitates subsequent data processing operations. On the other hand, use of an impulsive energy source can pose certain safety and environmental concerns.
Since the late 1950s and early 1960s, the second type of geophysical prospecting has developed, which employs a seismic vibrator (e.g., a land or marine seismic vibrator) as the energy source, wherein the seismic vibrator is commonly used to propagate energy signals over an extended period of time, as opposed to the near instantaneous energy provided by impulsive sources. Thus, a seismic vibrator may be employed as the source of seismic energy which, when energized, imparts relatively low-level energy signals into the earth. The seismic process employing such use of a seismic vibrator is sometimes referred to as “VIBROSEIS” prospecting. In general, vibroseis is commonly used in the art to refer to a method used to propagate energy signals into the earth over an extended period of time, as opposed to the near instantaneous energy provided by impulsive sources. The data recorded in this way is then correlated to convert the extended source signal into an impulse. The source signal using this method was originally generated by an electric motor driving sets of counter-rotating eccentric weights, but these were quickly replaced by servo-controlled hydraulic vibrator or “shaker unit” mounted on a mobile base unit. Roughly, half of today's land seismic data surveys use P-wave hydraulic vibrators for sources. Hydraulic seismic vibrators are popular, at least in part, because of the high energy densities of such devices.
The seismic signal generated by a seismic vibrator is a controlled wavetrain—a sweep signal containing different frequencies—that may be emitted into the surface of the earth, a body of water or a borehole. In a seismic vibrator for use on land, energy may be imparted into the ground in a swept frequency signal. Typically, the energy to be imparted into the ground is generated by a hydraulic drive system that vibrates a large weight, known as the reaction mass, up and down. The hydraulic pressure that accelerates the reaction mass acts also on a piston that is attached to a baseplate that is in contact with the earth and through which the vibrations are transmitted into the earth. Very often, the baseplate is coupled with a large fixed weight, known as the hold-down weight that maintains contact between the baseplate and the ground as the reaction mass moves up and down. The seismic sweep produced by the seismic vibrator is generally a sinusoidal vibration of continuously varying frequency, increasing or decreasing monotonically within a given frequency range. Seismic sweeps often have durations between 2 and 20 seconds. The instantaneous frequency of the seismic sweep may vary linearly or nonlinearly with time. The ratio of the instantaneous frequency variation over the unit time interval is defined sweep rate. Further, the frequency of the seismic sweep may start low and increase with time (i.e., “an upsweep”) or it may begin high and gradually decrease (i.e., “a downsweep”). Typically, the frequency range today is, say from about 3 Hertz (Hz) to some upper limit that is often less than 200 Hz, and most commonly the range is from about 6 Hz to about 100 Hz.
In many implementations, vibroseis technology uses vehicle-mounted vibrators (commonly called “vibes”) as an energy source to impart coded seismic energy into the ground. The seismic waves are recorded via geophones and subsequently subjected to processing applications. Today, various sophisticated vibrator systems are available for use, including minivibes, truck-mount vibes and buggy-mount vibes, any of which may be selected for use in a given application to provide the best possible solutions to meet a specific seismic program needs.
In seismic exploration, low frequencies (e.g., below 10 Hz) are particularly of interest today due, at least in part, to increased interest in performing acoustic impedance inversion. If seismic data can be obtained that is sufficiently quiet, then the acoustic impedance inversion process can be performed, which may result in some useful geotechnical information. An additional benefit of using low frequencies is that low frequencies penetrate farther than high frequencies, and so their use may permit evaluation of the Earth's subsurface at deeper levels. Further, by including some low frequency content in the data, it may help improve the continuity of reflectors and characteristics being imaged in the subsurface under evaluation.
In addition to the above-mentioned seismic exploration applications, active source signals are commonly employed for other applications, whereby the reflected portion(s) of such active source signals are processed for analysis of the reflector objects present in a target site. For instance, such active source signals are commonly used for medical imaging, acoustic location (i.e., using sound to determine the distance and direction of something), submarine navigation, ultrasound applications (e.g., medical imaging, such as sonography), etc.
For instance, in ultrasound-based medical imaging applications, a sound wave is typically produced by a piezoelectric transducer encased in a probe. Strong, short electrical pulses from the ultrasound machine make the transducer ring at the desired frequency. The frequencies are typically between 2 and 18 MHz. The sound is focused either by the shape of the transducer, a lens in front of the transducer, or a complex set of control pulses from the ultrasound scanner machine (through a beamforming process). This focusing produces an arc-shaped sound wave from the face of the transducer. The wave travels into the body and comes into focus at a desired depth.
Typically, materials on the face of the transducer enable the sound to be transmitted efficiently into the body (usually seeming to be a rubbery coating, a form of impedance matching). In addition, a water-based gel is placed between the patient's skin and the probe. The sound wave is partially reflected from the layers between different tissues. Specifically, sound is reflected anywhere there are density changes in the body: e.g. blood cells in blood plasma, small structures in organs, etc. Some of the reflections return to the transducer.
The return sound wave vibrates the transducer, and the transducer turns the vibrations into electrical pulses that travel to the ultrasonic scanner where they are processed and transformed into a digital image.
In applications that rely upon analysis of reflected portions of an active source signal (also referred to herein as “reflected signal analysis” applications), such as those discussed above, undesired reflections may occur that interfere with the desired reflections. For instance, is seismic applications, heterogeneity in oil and gas reservoirs presents obstacles to identifying reservoir rock and fluid properties. Multiple in-situ rock and fluid discontinuities cause undesired reflections that interfere with the desired reflections used by methods such as reflection seismology and sonar. Similarly, undesired reflections may interfere with desired reflections in medical imaging and other applications that rely on reflections of portion(s) of active source signals.